SOLVED - Cumulative Sum in Pivot tables

Hi - hope someone can help!
I have a lovely pivot table showing various sums by date, but I want to make these cumulative.
I have tried using the Running Total In option, but I'm still just getting actual values each month, not the cumulative amount. I[ve tried switching it from Budget/Actual field to Category field, still no cumulative total.
Where am I going wrong?
Attaching a sample workbook - the data set has been culled to only provide data for the specific project - normally there are hundreds of projects and 20k+ rows of data!
Many thanks in advance

In green the running totals.